This invention relates to an electrode retraction control system for controlling the retraction of an electrode in an electric discharge machine for subjecting a workpiece to electric discharge machining by impressing a machining voltage across the workpiece and an electrode travelling relative thereto.
Electric discharge machines include wire-cut electric discharge machines wherein a wire electrode is moved relative to a workpiece so as to be guided along a commanded path, thereby to perform electric discharge machining; and electric discharge machines wherein an electrode of a prescribed shape, held close to the surface of a workpiece, is moved into the workpiece to perform cutting and an electric discharge is produced across the electrode and the workpiece, thereby to subject the workpiece to machining in conformance with the shape of the electrode.
FIG. 1 illustrates schematically the latter electric discharge machine. In FIG. 1, an electrode EP acting as a punch is supported by a spindle SP and is fed for machining in the direction of the arrow by a servomotor, not shown. It is so arranged that a workpiece WK to be formed into a die and the electrode EP are dipped into a machining fluid and a square-wave voltage from a power source PS is applied across them. By advancing the electrode EP for machining while a spark discharge is produced across a minute gap between the electrode EP and the workpiece WK, the workpiece WK is subjected to machining in conformance with the shape of the electrode EP. The workpiece WK can be machined to have an enlarged bore of dimensions commensurate with the shape and amount of energy of the voltage pulses impressed across the electrode EP and the workpiece. When necessary an enlarged bore of desired size can be machined in the workpiece WK by moving the electrode EP in eccentric fashion.
In order to produce a satisfactory discharge across the electrode EP and workpiece WK, it is necessary that the gap between them be held constant at all times.
To this end, the prior-art practice is to use a synchronous feed system in which a voltage separate from the aforementioned machining voltage of square-wave shape is applied for sensing the gap between the electrode EP and workpiece WK, and the advance and retraction of the electrode EP are controlled automatically in accordance with a change in the gap sensing voltage, which corresponds to a change in the gap.
FIG. 2 is a view for describing this conventional synchronous feed system and illustrates a case where retraction is controlled.
When the electrode EP nears workpiece WK to form a gap in excess of a predetermined size, a gap sensing voltage Vd falls below a voltage Va that prevails when conditions are normal. If the gap sensing voltage Vd falls further and drops below a limit voltage Vb, the electrode EP is retracted relative to the workpiece WK. The travelling velocity of the electrode EP at this time is proportional to the gap sensing voltage Vd, as shown in FIGS. 2(a), (b).
When the gap between the electrode EP and workpiece WK is widened by retraction of the electrode EP to bring about a corresponding increase in the gap sensing voltage Vd, the voltage Vd eventually returns to the limit voltage Vb, retraction of the electrode EP is halted. This is followed by resumption of electrode advance. The velocity v during advance also is proportional to the gap sensing voltage Vd. When an advance velocity deceleration point is reached, the advance velocity v is reduced and again attempts to return to the point at which retraction starts. Such electrode retraction control is executed in accordance with a fixed sequence.
According to this conventional electrode retraction control system, however, a quick retraction operation cannot be achieved because the retraction velocity is dependent upon the gap sensing voltage. A result is continuation of an arc discharge that follows the spark discharge, causing the electrode EP and workpiece WK to collide in extreme cases. A problem that results is a deterioration in the precision to which the surface of the workpiece WK is machined. An additional problem is a reduction in the speed at which the workpiece WK is machined, this being ascribable to the excessive amount of time needed to exercise retraction control.